Why am I getting this cast error?

I have a Customer object in my project that extends the User object.

User.java

public class User {

private int userId;
private int user_type;
private String username;
private String password;

public User(int id, int user_type, String username, String password) {
    super();
    this.userId = id;
    this.user_type = user_type;
    this.username = username;
    this.password = password;
}

public int getUserId() {
    return userId;
}
public void setUserId(int userId) {
    this.userId = userId;
}

public int getUser_type() {
    return user_type;
}

public void setUser_type(int user_type) {
    this.user_type = user_type;
}

public String getUsername() {
    return username;
}

public void setUsername(String username) {
    this.username = username;
}

public String getPassword() {
    return password;
}

public void setPassword(String password) {
    this.password = password;
}

}

and my Customer.java

public class Customer extends User{

private String cartId;

public Customer(int id, int user_type, String username, String password) {
    super(id, user_type, username, password);
    // TODO Auto-generated constructor stub
}

public String getCartId() {
    return cartId;
}

public void setCartId(String cartId) {
    this.cartId = cartId;
}
}

Now, I created a simple login for my website that first checks of a user exists and if it does, it checks if its a customer or from the management.If its a customer, I have a method that fetches its cart id. Then I cast the user to customer type then set the cart id via setter.

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    // TODO Auto-generated method stub
    String username = request.getParameter("username");
    String password = request.getParameter("password");
    boolean userDoesNotExist = false;

    User user = ServiceFactory.userService().getUser(username);
    System.out.println(user.getPassword() + " " + user.getUsername());

    if(user != null){
        if(user.getUser_type() == 1){
            String cartId = ServiceFactory.customerService().getCartId(user.getUserId());
            Customer customer = (Customer) user;
            customer.setCartId(cartId);

            request.getSession().setAttribute("customer", customer);
            response.sendRedirect("customer");
        }else{

        }
    }else{
        userDoesNotExist = true;

        request.setAttribute("userDoesNotExist", userDoesNotExist);
        request.setAttribute("username", username);
        RequestDispatcherManager.dispatch(this, "/login.jsp", request, response);
    }
}

So, back to my question, why am I getting a ...

java.lang.ClassCastException: com.qbryx.domain.User cannot be cast to com.qbryx.domain.Customer
com.qbryx.servlets.LoginServlet.doPost(LoginServlet.java:50)
javax.servlet.http.HttpServlet.service(HttpServlet.java:648)
javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)